Power switches, such as insulated gate bipolar transistors (IGBTs), include single or multiple silicon dice. The peak transient voltage on these dice is a critical design limit. A device, such as an IGBT, has, for example, two primary ratings, one a voltage rating and other a current rating. For economic and performance reasons, it is typically desirable to make as full use as possible of the available voltage rating and the available current rating of a device. A device with a voltage rating, for example of 600 volts, is typically run at a much lower bus voltage or working voltage because of parasitic components. As a rule of thumb, in most applications the working voltage is set to be approximately half of the rated voltage. Accordingly, the full voltage rating of the device is typically not fully utilized.
It is noted that a steady direct current (DC) voltage supply to an inverter is not critical. Such a DC voltage may come from a variety of DC voltage sources such as batteries, fuel cells, and/or ultra-capacitors to name a few. A typical DC voltage supply may supply, for example, 300 volts to the inverter. Currently, a 600 volt rated die may typically be used in such an inverter design. The reason for using, for example, a 600 volt rated die with a 300 volt power supply is that switching the conducting current in the inverter ON and OFF causes additional voltage spikes on the die. Energy loss and heat build-up in the device are associated with switching the device OFF too slowly, so these devices are designed to switch OFF as quickly as possible. Typically, when the device is switched OFF, the voltage spikes momentarily and then falls back. Although the voltage spike may last only a microsecond, the silicon of the die may suddenly and catastrophically break down under such a voltage spike. Even a microsecond of over-voltage is enough to destroy the device.
While the peak die voltage is a crucial operating parameter for power semiconductors, the peak voltage cannot be directly measured on an off-the-shelf product. While the voltage that is measured on the outside of the device at the terminals may be 600 volts, the actual voltage inside the device will always be greater than at the terminals. Knowledge of the actual or true die voltage, indicates how much margin exists prior to breakdown.
There is a need for a way to measure this voltage on the silicon in order to fully utilize the device. One way to determine whether the voltage rating is being fully utilized is to determine the voltage on the silicon itself, because it is the silicon itself that is actually rated. However, it is very difficult to determine the voltage on an actual die because the dice are fragile and carefully protected. Usually, such a determination requires destruction of the protective packaging surrounding the die, for example by cracking open the protective packaging, and probing the particular points directly on the die. The destructive nature of this testing process has at least three distinct drawbacks. The first drawback is that the destruction of the protective packaging renders the die unusable for commercial applications. The second drawback is that only the statistically determined theoretical limit for a type of die can be determined—the actual limit for a particular die to be used in a commercial application cannot be determined without rendering the die unusable. The third drawback is that the limit of a particular die cannot be monitored during actual use of the die in a commercial application (i.e., “in the field”), and thus cannot be periodically or continually monitored, for example as the die ages.
Especially in the case of commercially available power transistor switches, the only accessible measurement points are at the external terminals. Since the voltage inside the device is unknown, the problem is typically addressed by reducing the operating voltage and/or making the switching slower. In that way, less power is available from the hardware and less energy loss and heat build-up occurs. Those skilled in the art will recognize that when switching is slowed down, the amount of current for which the customer pays is reduced for an equivalent thermal load. Thus, while it is important to utilize as much of the rated voltage of the device as possible, to do so requires methods and apparatus that non-destructively and non-intrusively determine die voltage based on terminal properties.